In a projection display, light from a light source has an image imparted to it and is then projected onto a screen for viewing. The display may be a front projection one, in which the projection mechanism is positioned on the same side of the screen as the viewer, or a rear projection one, in which the projection mechanism is positioned on the side of the screen away from the viewer. To achieve a colored display, three colored light beams (red, green, and blue), each having a respective red, green and blue image may be combined and projected.
The red, green and blue light beams may be generated from separate red, green, and blue light sources. However, using a single white light source whose light is separated into red, green and blue beams is a preferred design, for technical and economic reasons. The separation (and subsequent recombination) may be effected by a series of dichroic mirrors, such as taught in Williams et al., WO 90/05429 (1990), and Tanaka et al., U.S. Pat. No. 5,164,821 (1992).
The use of mirrors is undesirable from the point of view of compactness. It has also been taught to use a dichroic cube or prism for separating the white light into the colored components or combining the colored components, or both, as dichroic cubes or prisms can be made relatively compact. Illustrative teachings include Masanori et al., EP 0,362,776 A3 (1990); Sato et al., U.S. Pat. No. 5,097,323 (1992); Sonehara, U.S. Pat. No. 5,098,183 (1992), Sato et al., U.S. Pat. No. 5,105,265 (1992); Baur et al., U.S. Pat. No. 5,115,305 (1992); Kurematsu et al., U.S. Pat. No. 5,170,194 (1992); and Plantier et al., U.S. Pat. No. 5,172,222(1992).
The prior art employs three separate imaging elements for imparting images to each of the red, green, and blue light components, as typified in the Masanori et al. published application and the Sonehara and Kurematsu et al. patents. These imaging elements often are liquid crystal cells, especially of the twisted nematic ("TN") type. Such a design suffers from a number of limitations. An imaging element is among the more expensive components of a projection display, especially in a high resolution video display, because of the large number of pixels which the imaging element must contain, along with means for addressing them. Thus, it is desirable to have a projection display which does not require separate imaging elements for the red, green, and blue light components. Also, where the imaging element is a TN cell, the requisite polarizer extracts a price in the form of the inevitable loss of 50% of the incident light, with a concomitant decrease in brightness. (While one can compensate for the decrease in brightness by using a more powerful light source, this undesirably increases power consumption and heat output.)
A color projection display of the present invention offers a number of improvements over the prior art, including the ability to operate with a single imaging element and the ability to operate in the absence of polarizers.